The present invention relates to an intake system for an internal combustion engine having primary and secondary intake systems each having a carburetor, in which the primary intake system operates for light load and the secondary intake system operates for heavy load.
In order to improve combustion efficiency, a turbulence-generating port for providing a swirl effect is known. The turbulence-generating port is curved so as to admit air-fuel mixture in a tangential direction. However, such a curved port increases the resistance in the port.
As a method for improving the output at heavy load in addition to the swirl effect, Japanese Utility Model Publication 56-50132 discloses an intake system for an automotive engine comprising a first intake system for light and partial loads and a second intake system for heavy load. Each intake system has an intake passage having a carburetor and an intake port separated from the intake port for the other intake passage. At light and partial load, mixture is admitted into a corresponding cylinder through the first intake system, and at heavy load, mixture is admitted through both systems. The intake port of the first intake passage is reduced in cross sectional area for increasing the velocity of the mixture and disposed in a tangential direction to provide a swirl effect.
In such a system, the swirl effect or swirl force is constant during light and partial load operations, because the sectional area of the intake port is not changed in accordance with load on the engine. By the way, it is desirable that swirl effect varies with the load on the engine and with engine speed in order to provide optimum combustion efficiency. Since combustion is not efficiently conducted at low engine speed and light load, high swirl effect is required. In other words, it is desirable to increase the swirl effect as engine speed and load decrease.